The search for environmentally responsible disposal techniques of waste products has been problematic in the past, and is likely to continue to be a significant issue in the future. The United States Environmental Protection Agency (“EPA”) in its February 1989 “Agenda for Action” estimated the annual generation of municipal solid wastes will increase from 160 million tons in 1988 to 193 million tons by the year 2000. Further, many industrial wastes are not amenable to thermal, chemical, or biological destruction. When these wastes cannot be reused or recycled in any beneficial, economical way, there is no other recourse but land disposal or landfilling. The decision to landfill waste depends on the characteristics of the waste, complicated environmental regulations and economic considerations.
Roughly half of the Electric Arc Furnace (“EAF”) dust generated today is destined for a landfill. The composition of EAF dust, especially dust with low levels (less than about 15%) of zinc, makes it costly to recycle or to dispose of by conventional non-landfill methods. Since these alternatives are not economical, the default alternative, landfilling, is often the method of choice. Currently, it costs approximately $100.00 to $150.00 a ton to process and dispose of EAF waste. EAF dust is generated at 0.4 to 3.2% of the steel produced. This represents, on average, an additional $2.25 per ton for every ton of steel manufactured.
Another waste source arises when ferrous metal products are commonly rendered corrosion resistant by the application of electroplate coatings of non-ferrous metals such as zinc, nickel, copper, cadmium, and chromium. Zinc is the metal which is predominantly employed for electroplating. Presently, more than 40% of the plating shops in the United States electroplate zinc from a variety of plating baths.
Each year, zinc plating operations in the United States generate five billion gallons of waste water contaminated with zinc, which must be removed prior to release of the waste water into the environment. The majority of zinc platers currently employ conventional waste water treatment techniques for the removal of zinc which yield a toxic metal sludge. Even when highly concentrated, this sludge is not reusable in plating baths, since it contains substantial amounts of iron and the water hardness factors, calcium, and magnesium salts. Therefore, the sludge must be packaged in containers and shipped to environmentally secured landfill sites after chemical treatment. The total cost of such safe, permanent disposal is very high. In fact, it can equal or exceed the value of the chemicals used in the plating process.
Most of the costs associated with the disposal of these industrial wastes are a result of regulations promulgated to protect the environment. Generally speaking, these regulations are concerned with the containment of the landfilled waste. The most significant concern in landfill waste containment is potential leaching of the waste from its containment area into migrating water.
Leaching occurs when ground or surface water (migrating water) contacts or passes through a material and dissolves a constituent of the material at some finite rate. The water, prior to passing through the material is called leachant and the contaminated water that has passed through the waste, the leachate. The capacity of the waste material to leach is called its leachability. The concern over leaching for the environment relates to the rate at which hazardous and other undesirable constituents are removed from the waste and the environment via the leachate. This rate is usually estimated and expressed, however, in terms of concentration of the constituents in the leachate derived from a given test method. Concentration in the leachate is important because it determines the constituents' effects on living organisms, especially humans. When evaluating a material for leachability, the concentration of the hazardous constituents in the leachate are compared to either a specific concentration amount or to that in the original waste or both.
Material solubility and migrating water greatly influence the fate of compounds in the environment and therefore leachability. Highly soluble compounds readily distribute by way of the hydrologic cycle, do not bind readily to sediment, and bio-accumulate less rapidly than less soluble lipophilic chemicals. Factors influencing solubility include the degree of ionization of the compound, its oxidation state, the pH, and mineral content of the medium. Therefore, the behavior in the environment depends, in part, upon whether a hazardous compound or element is in an oxidized or non-oxidized state.
Some individual metal elements may have many different oxidation states; they are designated as metals because of the characteristics they display as a pure phase. In the zero oxidation state, each metal element is electrically neutral, however it may readily share electrons with surrounding atoms. The resulting solid may conduct electricity, or have a shiny appearance, and often has the great strength that makes metals such useful materials. Metals are oxidized when electrons are removed from the atoms (typically by molecular oxygen, hence the name for the process). The resulting ion may become part of a crystalline oxide, or may dissolve in water. These ionized species are often soluble in water because water molecules are electric dipoles: one end has a slight negative charge, and one is slightly positive. Thus, a chunk of zinc metal is immobile in the soil, but may dissolve as oxidized zinc ions which migrate easily into the leachate if exposed to an oxidizing agent.
Alternatively, ions may gain electrons. This is called reduction because the oxidation state of the compound or element is being reduced. For example, a species of chrome may carry a 6+ charge, Cr6+ oxidation state. When the species is in an electron rich environment, it may gain electrons. The addition of the electrons to the 6+ charge of chromium results in a 3+ state, hence the 6+ is lowered, or reduced, to a 3+ oxidation state.
Just as pH is a measurement of the concentration of protons in solution, the oxidation reduction potential (ORP), electrode potential or redox potential is a measurement of the availability of electrons that play a role in oxidation or reduction. Where electron-donating chemicals are abundant and electrons are readily available, the ORP is low. Where electron acceptors are numerous and powerful, the ORP is high.
Because atmospheric oxygen is a strong electron acceptor, its presence raises ORP values. In contrast, sulfur containing compounds and organic chemicals, particularly during decomposition by microorganisms, are usually electron donors and create environments with low ORPs. This close association with oxygen concentrations has led to indirect characterization of the ORP in terms of oxygen availability. “Aerobic” and “anaerobic” conditions are associated with high and low ORP values respectively. FIG. 1 demonstrates this characterization.
There is a growing substantive body of evidence demonstrating that landfills receiving solid wastes are anaerobic or oxygen depleted as a result of their formation and maturation (U.S. EPA, 1989), (Farquhar, G. J., 1988). There are several typical steps in the maturation of landfills. FIG. 2 is a schematic of the theoretical life cycle for a typical solid waste landfill that shows the relationship between aerobic and anaerobic environments, along with related measurement parameters (Mara, K. and Burlaz, M. A., 1987).
Generation of methane from the degradation of organic materials at landfills is strong evidence that a landfill is anaerobic or oxygen depleted because microorganisms responsible for producing methane from respiration are only capable of surviving under anaerobic conditions. In fact, subtitle D landfills (most municipal solid waste landfills), routinely monitor for methane and have systems in place to effectively manage the methane generated. Moreover, the U.S. EPA has a program in place to encourage the use of methane as an energy source.
The generation of methane, however, is not an exclusive indicator of oxygen depletion or anaerobic conditions. Other reduction reactions, such as the conversion of sulfate to sulfides and nitrates to nitrogen are also indicative of oxygen depleted environments.
The formation of anaerobic environments with low oxidation reduction potential is a function of the availability of oxygen and bacterial activity that consumes oxygen (Brach T. D., 1970). FIG. 1 schematically shows the relationship between anaerobic and aerobic conditions in the environment and ORPs (modified from Bouwer, 1994). Similar to landfills, certain natural environments such as lake sediments, bogs, and marshes are often anaerobic and characteristically exhibit low ORPs.
While land disposal of hazardous waste is not the method of choice from an environmental standpoint—it occupies the lowest position in EPA's hierarchy of methods—it has nonetheless developed an important place in the overall waste and management scheme. This waste management is dictated by complex environmental laws and regulations. The most important of these are contained in the Resource Conservation and Recovery Act (“RCRA”). RCRA dictates when, how and why wastes are disposed. According to RCRA regulations, wastes containing lead, chromium, cadmium, arsenic, mercury, selenium, silver, or barium, must leach no more than minimum concentrations of these metals to be legally defined as non-hazardous. Pursuant to RCRA and 40 C.F.R. part 261, wastes generally are defined as hazardous in two ways. First, wastes are hazardous wastes if they are listed by EPA as hazardous in 40 C.F.R. part 261 subpart D. Second, wastes are also hazardous if they exhibit any of the four characteristics found in 40 C.F.R. part 261 subpart C. These characteristics are ignitability, corrosivity, reactivity, or toxicity based on the Toxicity Characteristic Leaching Procedure (“TCLP”).
Pursuant to the EPA, the TCLP test must be used to determine compliance with the leachable metal concentration allowances. The TCLP test is described in 40 C.F.R. § 261, appendix II, which makes reference to SW-846 method 1311. Wastes that fail the TCLP test are hazardous and must be treated and disposed of in permitted RCRA facilities. The disposal of wastes in these facilities is expensive.
This demonstrates the central role the TCLP test plays in waste disposal because it dictates the “hows”, “whys” and “wheres” mandated by RCRA. It follows that, to dispose of waste efficiently and inexpensively, the waste must be non-hazardous; to be considered non-hazardous the waste must pass the TCLP test; only then can the waste be taken off the hazardous list, or delisted.
With this understanding of the important environmental characteristics of landfills, the wastes contained therein and the regulations that govern their use as a backdrop, many have attempted to treat or stabilize waste prior to landfilling in order to prevent leaching. The processes and techniques of stabilization/solidification (S/S) have matured into an accepted, important part of environmental technology. As a result, a great many S/S methods have been promoted recently and offered for the treatment of hazardous and other wastes from industry, municipalities, and government sources.
The waste management field has long felt the need for processes that remove and retard metal leachability to render hazardous waste non-leachable. Typically encapsulation processes are employed to reduce the chemical mobility of wastes, whereby various encapsulants are added to the wastes (i.e., liquids and semi-solids) to turn the waste into a solid. Examples of such encapsulants are cement, sodium silicate, asphalt, glass, and various polymers and chemicals. For example, U.S. Pat. No. 5,916,123 to Alexander discloses a process for vitrifying waste into a stabilized form. This process basically solidifies the waste with silicon oxides or incorporates the waste into glass. A Critical Review of Stabilization/Solidification Technology, by Conner and Hoeffner, discloses the accepted practice of incorporating metal bearing hazardous materials into cement.
Unfortunately, encapsulation processes have not completely resolved the problem of leachability. Moreover, cement and glass bound wastes are porous, which allows water to penetrate the encapsulated waste. Water, of course, is prone to alternately expand and contract during freeze-thaw cycles. If the treated waste is exposed to freezing temperatures, this expansion and contraction may cause the solidified waste to crumble and expose even more surface area to water, which can further leach the wastes into the environment. Further, these solidification processes tend to be expensive and often greatly increase the amount of material for disposal.
Other traditional heavy metal bearing waste treatments use lime or NaOH to precipitate the metals as hydroxides. For example, U.S. Pat. No. 4,671,882 to Douglas discloses a treatment of metal bearing waste at high pH. U.S. Pat. No. 5,916,123 to Pal discloses the use of lime, or gypsum to treat metal bearing waste. These treatments are based upon the theory that converting hazardous metals to hydroxide form, causes the waste to precipitate and become less soluble in the water leachant. To a large extent this is true. However producing metal hydroxides from mixed metal waste does not ensure the waste is non-leachable. Each metal has an optimum pH range in which the metal hydroxide is non-leachable, some metals at higher pH and others at lower pH, making it difficult for all metal hydroxides to be equally non-leachable at a specified pH.
Sulfide has also been utilized to treat primarily aqueous wastes. (Environmental Protection Agency Summary Report on the Control and Treatment Technology for the Metal Finishing Industry—Sulfide Precipitation, April 1980). However, the use of sulfide alone to precipitate metals in aqueous wastes is problematic because the amount of sulfide cannot be determined accurately, and typically the use of sulfide to precipitate requires excess amounts.
Therefore a need still remains for processes for treating wastes containing hazardous materials that result in the production of non-leachable materials that can be safely and economically disposed of in a landfill.
Additionally, a need remains for waste analysis methods that accurately simulate the practical conditions of landfills so that the leachability of wastes slated for landfill can be correctly predicted. Such accurate methods would protect the environment and promote business that uses environmentally sound methods.